The center wire (anode) is maintained at a high positie voltage relative to the outside cylinder (cathode). The tube, with a thin window at one end, is sealed and filled with a special gas mixture, typically argon and a quenching gas. When ionization occurs, electrons are accelerated toward the center wire. Gas amplification occurs. In addition to ionizing gas molecules, the accelerating electrons cause excitation of gas molecules through collisions. The excited molecules quickly return to ground state emitting UV radiation. UV photons interacts via photoelectric absorption releasing another electron, triggering an avalanche. This is process is terminated by the positive ions that reduce the effective electric field around the anode. A constant electric charge is created that is large and easily detected with electronic circuits.